1. Technical Field
Aspects of the present invention relate to image processing technology and, more specifically, relate to a method and an apparatus for correcting an image signal generated by an image sensor, such as, a charge coupled device (CCD) of an imaging system, and the imaging system having such a charge coupled device (CCD) image sensor.
2. Description of Related Art
In general, image sensors are used in imaging systems, such as, for example, digital cameras, optical scanners and video cameras as light sensing devices to convert a visual image into an electrical signal. An image sensor is typically an array of photoelectric cells, also known as picture elements (i.e., pixels), arranged in a matrix form so as to convert the light energy into electrical signal charge and subsequently output as an image signal, when a visual image is projected thereon. Each of the pixels, which form the visual image, is an optical sensor that is adapted to sense an incident light beam and photo-electrically convert the light beam into electrical signal charge corresponding to the amount of the received light beam. Such an image sensor can be any type of image sensor, such as a charge coupled device (CCD) or complementary metal-oxide semiconductor (CMOS) device. CCD image sensors can be implemented in several different architectures, including, for example, full-frame, frame-transfer and interline techniques depending upon usage and applications.
One of the challenges of utilizing such an image sensor in an imaging system is to reduce image distortion and to minimize any loss of image details in a dark region or a bright region of an image signal. For example, FIG. 13 illustrates a gray scale histogram of an image signal describing a visual image, such as, an object scene with a brightness range beyond a dynamic/effective range of the imaging CCD. As shown in FIG. 13, the gray scale histogram of an image signal is represented by a pixel amount as a function of a gray scale using Cartesian coordinates. Specifically, on the gray scale axis (i.e., X-axis) as a function of the pixel amount axis (i.e., Y-axis), “0” denotes pure black, and “D” denotes pure white. The range 0˜D is the dynamic range of the CCD. An area that is located in the lower part of the range 0˜D represents a dark portion of the image signal (i.e., shadows). Conversely, an area that is located in the higher part of the range 0˜D represents a bright portion of the image signal (i.e., highlights). The histogram contour curve 10, as shown in FIG. 13, indicates: (1) any portion of the image signal brighter than “D” will be displayed as pure white by the pixels corresponding to that portion losing any detail that would be revealed with different degrees of whiteness displayed; and (2) any portion of the image signal blacker than “0” will be displayed as pure black by the pixels corresponding to that portion losing any detail that would be revealed with different degrees of blackness displayed. In other words, the CCD image sensor can not describe the bright portion of an object scene in detail. Additionally, the pixels corresponding to the brightest portion(s) of an object scene may bloom, inducing a smear effect in the CCD image sensor. As a result, characteristics of an image signal may distort. Likewise, the CCD image sensor can not describe the dark portion of the object scene in detail as well.
In order to avoid inducing the smear effect in an image, a CCD image sensor is typically exposed to a smaller amount of light intensity. As result, less image details are lost in the bright portion of an image signal. However, a loss of image details in the dark portion of the image signal is much more severe. Referring to FIG. 14, when a CDD image sensor is exposed to a smaller amount of light, less details are lost in the bright portion of the image signal, but a loss of details in the dark portion of the image signal is severe, as indicated by the histogram contour curve 20.
Therefore, it is desirable to provide a method and an apparatus for correcting an image signal generated from an image sensor, such as a CCD image sensor, and an imaging system incorporating such a CCD image sensor so as to reduce image distortion and to minimize any loss of image details in a dark region or a bright region of an image signal.